This invention relates in general to transmissions for transmitting rotary motion and, more particularly, to a method for power flow management in an electro-mechanical output power-split and compound power-split infinitely variable transmission.
The primary function of a transmission in an automotive vehicle is to convert the output speed and torque of an engine or prime mover into the speed and torque of a drive shaft to meet the specific requirements of the vehicle. In general it is desirable for this power transmission to occur with minimal power loss; that is, at high transmission efficiency. To optimize the overall power train performance, specifically the engine and transmission combination, it is also desirable to minimize engine transients and operate the engine at a power state that yields the best tradeoff between good fuel economy, low emissions and power.
Transmissions can be broadly categorized as “step-less” or “stepwise”, according to the way that the final desired speed ratio is achieved. A stepwise transmission has a fixed number of speed ratios that are sequentially selected to achieve the final desired ratio. Therefore, it is not always possible to use the maximum available engine power, because engine speed and load vary as the transmission moves through its limited number of fixed ratios. Moving from one ratio to another is sometimes termed “shifting” in transmission design descriptions, and is often associated with unfavorable torque interruptions. Such interruptions offset overall drive-line efficiency and detract from driving comfort. Moreover, for some applications, such as agricultural and construction vehicles, an output torque interruption is extremely undesirable, more so than a momentary loss of efficiency. Such inherent shortcomings, associated with shifting, are minimized to some extent in more recent designs that incorporate pre-shifting and clutching to reduce torque interruptions. To reduce engine speed and torque variations, modern transmissions use a greater number of selectable ratios.
In principle, a stepwise transmission cannot achieve the overall vehicle efficiency of a step-less transmission which offers an infinite number of speed ratios. With a step-less transmission, there is the option to operate the engine at its optimum efficiency or lowest emission point at all times while the vehicle is moving to its desired speed at a desired power level.
Step-less transmissions can be further classified into continuously variable transmissions (CVTs) and infinitely variable transmissions (IVTs). A continuously variable transmissions, usually mechanical, provides continuously variable speed ratios over the designed speed range of the vehicle. A launch-clutch and engine-disconnect device is often required in this type of transmission for vehicle start-up, as well as a separate gear for reverse operation. In addition, most of the CVT designs transmit torque through contacting friction surfaces and are not suitable for high-torque and high-power applications.
The infinitely variable transmission, by definition, is capable of providing infinitely selectable (output-to-input) speed ratios from reverse, thru zero, to a wide range of forward speeds. No launching device is required for an IVT. In theory, the engine can be directly connected to the transmission at all times, because a zero output-to-input speed ratio or an infinite input-to-output speed ratio exists. An IVT transmission is based on a power-split concept that provides multiple, often hybrid, power paths. Among the multiple power paths, there exists at least one power path that regulates the power flow and thus provides the input-to-output speed ratio controlling path. The actual devices that effect the speed ratio change within the regulating path is called a variator. A well designed power-split transmission is able to transmit power from the input to the output in at least two parallel power paths, one path being the regulating or variator path, and one path being a mechanical path. While the speed ratio is controlled in the variator path, it is most desirable to have the majority of the power pass through the mechanical path were the efficiency is the highest.
The speed ratio change is made by the variators by regulating the power that is transmitted through the variator path. There are moments where the power that passes through the variator path is literally zero. That is, all power is transmitted through the mechanical path. At these moments, the transmission yields the highest efficiency. The transmission output-to-input speed ratios or the output speeds at these points are often referred to as node points or nodes. The node point corresponding to the lowest output-to-input speed ratio or the lowest output speed is called the first node point. The node point corresponding to the next higher output-to-input speed ratio or the next higher output speed is the second node point, etc.
In an electromechanical infinitely variable transmission (eVT), the variators are electric machines, namely motors and generators. The advantages of using electric motors and generators include system design flexibility, controllability, and improved performance in terms of a wider range of speed, better efficiency, and reduced operating noise. The electromechanical transmission may also be used in conjunction with energy storage devices to supplement the output power, allowing the engine to be downsized and/or to operate at an optimum efficiency point for a greater period of time. These so-called hybrid electro-mechanical power-split transmissions have been recently designed for use in both cars and in heavy trucks.
There are three basic power-split configurations, the “input power-split”, the “output power-split”, and the “compound power-split”. These three configurations define different relationships between the power within the variator path and the speed ratio of the transmission. Input power-split and output power-split configurations are often devices with single planetary transmissions, and are capable of providing at least a single node point. Compound power-split configurations are associated primarily with compound planetary transmission, and are capable of providing at least two node points.
Within power-split transmissions, there are moments when the speed ratio goes beyond a certain range and the power in the electric path can actually exceed several times the power that is transmitted through the mechanical path in the transmission. This phenomenon is known as internal power circulation. It consumes power and decreases transmission efficiency by generating unwanted heat within the transmission.
For input power-split transmissions, internal power circulation occurs when output-to-input speed ratio is somewhere below the node point. When the transmission is operated in the slow speed regime, power in the variator path can actually exceed several times the power that is transmitted through the transmission. Likewise, internal power circulation can occur with output power-split transmissions when operated at high output-to-input speed ratios (overdrive), somewhere above the node point, or in reverse operation. Compound power-split transmissions have very high efficiency when operated between the two node points. Internal power circulation occurs at speed ratios somewhere below the first node point or above the second node point.
To restrict power circulation through the electric machines and to control the magnitude of the electric power in the electric power path, various control systems are known, for example, as shown in European Patent No. 0 867 323 A2 and in U.S. Pat. Nos. 5,907,191, 5,914,575, and 5,991,683 for an output power-split configuration and an input-power-split configuration. These control systems employ a torque control routine to monitor the electric motor torque and to shift the engine's operating point to a higher engine speed when the torque limit of the electric motor is reached. However, in doing so, the engine is operated at a point which is less than optimal.
An alternate control system is shown in U.S. Pat. No. 6,478,705 B1 to Holmes, et al., the '705 patent. This '705 patent discloses an eVT having two differential gearsets coupled to an engine and first and second electric machines, wherein the gearsets are configurable in input-split and compound-split modes. The control system disclosed in the '705 patent provides for shifting between the input-split and compound-split modes of operation only at a zero speed point of one of the electric machines, thereby permitting smooth synchronous clutch engagement. However, since the mode shift in the control system of the '705 patent occurs independently of the state of the second electric machine, the problem of internal power circulation remains when the transmission is operated in reverse, and when the second electric machine continues to provide torque to the system during a mode shift.
Accordingly, it would be advantageous to develop an eVT which does not suffer from the above-mentioned internal power circulation problem when operated within the designed speed ration ranges specified by a vehicle application.